Jennifer Lopez yells. She's standing on top of a riser, in a warehouse turned rehearsal studio tucked deep in California's San Fernando Valley, looking out at an audience of half a dozen people and holding a microphone in her hand. From a distance she is all hair and heels; the softness of her flowing, caramel-colored locks is a perfect foil to her spiky 6-inch gladiator stilettos, which feature no fewer than two dozen straps from toe to knee. (She usually performs in kitten heels, but today she's test-driving these bad boys to see if she can move in them.) Otherwise dressed down in leggings and a cropped white jacket over a black sports bra, she glides down the stairs to the center of the floor, her entrance announced by her band's drums and trumpets.

For the next hour Lopez is a blur, twisting and twirling and jumping and shimmying across the makeshift stage as the music throbs along, her tousled hair flying, obscuring her face except for her glossy red lips, which alternate between sly smile and pronounced pout. If her movements are even slightly hampered by those heels, it is not obvious.

Lopez is rehearsing for a surprise performance she will give in two days at Vancouver's We Day, an annual event that rewards kids who volunteer in their communities with a daylong festival of speakers and music acts. After running a few times through a 10-minute medley of some of her greatest hits, she tosses her jacket aside to reveal abs that seem to defy the laws of physics and gravity, not least because they're on the body of a 45-year-old mother of two. Since Jennifer Lopez the person evolved into J.Lo the brand nearly two decades ago, it's easy to forget that she started out as a member of the Fly Girls, the dance troupe featured on the sketch-comedy show In Living Color, which also launched the careers of actors Jim Carrey and Jamie Foxx. Though she's mostly thought of as a singer and an actress now, Lopez is fundamentally an athlete, and she approaches her life with the same kind of intense kinetic energy that informs her choreography.

"I still consider myself a dancer first and foremost," she says. "Well, after being a mother, of course." Though Lopez already put in enough rehearsal time last weekend to need only a light walk-through tonight, she's here because she doesn't believe in doing anything halfway. She goes hard, in stilettos, on a Monday night, because that's who she is. Between swigs of water, Lopez listens intently as her 10 dancers, many of whom have been with her for years, chime in with stage ideas for the kids' show, including massive sets of feathers and dramatic mic tosses, before giving them her own tips in calm, direct shorthand. "That's right, you've gotta hit it here," she says to one of the girls as she demonstrates a hip twerk. "And let's get in a straight line for that last part," she says to the guys. The dancers bounce back into place for another go, smiling because—well, because they get to dance with Jennifer Lopez for a living.

Lopez is smiling, too, even though it's been a long day. Earlier, she filmed an hour-long interview with Maria Shriver for the Today show. Then she drove for an hour to attend a taping of Dancing With the Stars, in support of her close friend, the actress Leah Remini, who was cohosting. When she is done with her dance rehearsal, at around 10 p.m., she will head into a studio to record a few new tracks with Rodney Jerkins, the prolific producer behind "If You Had My Love," her first hit from 16 years ago, and the Destiny's Child smash "Say My Name," among others. In spite of her hectic schedule, she swears she sleeps eight hours a night, joking that she's too boring to be found out on the town after midnight. "I'm in bed between 11:30 and 12:30 unless I'm pulling an all-nighter or on a night shoot," she says, which also might help explain why her complexion looks even more flawless in person.

It's a good thing Lopez is catching up on her rest, because 2015 is going to be a huge year for her. In addition to her day job as the good-cop judge on American Idol, Lopez will partner with the folks at health and fitness company BodyLab to create a new program for women, complete with recipes, health tips, smoothies and calorie and exercise trackers; she'll star as a police detective who is also a single mother in the upcoming television drama Shades of Blue; and she'll release two movies and continue to make music. All while working on another important project: herself. "I want to prove to everyone that I can do everything and be a superwoman," she says. "But I have to take time for myself."

The Boy Next Door, one of Lopez's upcoming films, opens later this month and is a bit of a passion project for her. Made on a shoestring (by Hollywood standards) budget of $4 million, the fun, erotic thriller features Lopez as Claire Peterson, a suburban mom with a philandering spouse who is seduced into a one-night stand by the 19-year-old heartthrob who lives next door. Lopez's character realizes her mistake and says as much to the boy, played by newcomer Ryan Guzman. Guzman's character spends the rest of the movie doing his best Glenn Close in Fatal Attraction.

When I ask Lopez what drew her to the character of Claire, her almond-colored eyes widen. "I can relate," she says. "I've been through divorce. I've been cheated on, just like every other girl in the world. So you sympathize, you understand the emotions. And I've dated a younger guy once in my life, so I could definitely understand that part, too—the attraction."

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